A Ziegler-Natta catalyst that is widely used in commercial processes is a multi-site catalyst, and thus produces polymers with a broad molecular weight distribution and an uneven comonomer distribution. Therefore, it is difficult to obtain desired physical properties.
On the contrary, a metallocene catalyst is a single-site catalyst having only a single kind of active site, and thus produces polymers with a narrow molecular weight distribution. Also, according to structures of the catalyst and ligand, molecular weight, tacticity, crystallinity, and in particular, comonomer reactivity, can be greatly controlled. However, a polyolefin polymerized using the metallocene catalyst has inferior workability due to a narrow molecular weight distribution, and in particular, has significantly lowered producibility when applied to some products due to the effects of extrusion load. Thus, there have been many efforts to control the molecular weight distribution of polyolefins.
For example, U.S. Pat. Nos. 4,461,873 and 7,250,473 suggest that polyolefins having different molecular weights and comonomer contents are physically mixed to control the molecular weight distribution. In addition, U.S. Pat. No. 7,345,113 suggests that the molecular weight distribution is controlled by varying polymerization conditions in each reactor of a multiple reactor system. The former method is problematic in that the physical mixing requires additional production costs, and a large amount of gel is contained due to compatibility of the two polymers to reduce physical properties. The latter method is also problematic in that the process is complicated, there is a limitation in broadening the molecular weight distribution because of limited hydrogen reactivity of the catalyst, and the polymerization conditions are also complicated.
Another known method for preparing a polyolefin having a wide molecular weight distribution is to use a Ziegler-Natta mixed catalyst, but this method also has a problem of requiring two or more reactors.
In addition, U.S. Pat. Nos. 6,444,605, 6,399,531, and 6,399,723, and Korean Patent Publication Nos. 1999-022334 and 2000-0042620, disclose that two different catalysts are blended in a single reactor to perform polymerization or two or more catalysts supported by a single support are used to perform polymerization. However, most of them mention a catalyst mixture including a metallocene compound and a Ziegler Natta-based titanium metal compound supported by a single support.
Therefore, there is a need to develop a method for preparing a metallocene catalyst capable of providing a polyolefin having a wide molecular weight distribution for the improvement of mechanical properties and processability.